1. Technical Field
The invention relates to a system for energy optimization in a plant for producing direct-reduced metal ores, especially direct-reduced iron. In this case a plant for producing direct-reduced metal ores or metals, especially iron, has at least one reduction unit (e.g. fluidized bed reactor system, packed bed reduction shaft such as e.g. MIDREX® reduction shaft, etc.), a device for separating gas mixtures having an associated compressing device and a gas heating device arranged upstream of the reduction unit. The process gases necessary for producing direct-reduced metal ores are at least partly supplied via a feed line from at least one plant for pig iron production such as e.g. a smelting reduction plant and are possibly partly obtained by recycling from the production process itself.
2. Prior Art
The so-called direct reduction of metal ores or metals, especially of iron or iron oxide, to direct-reduced iron or sponge iron, is carried out for example in a separate plant—known as a direct-reduction plant. Such a plant for producing direct-reduced metal ores usually comprises at least one reduction unit such as a packed bed reduction unit for example, a fluidized bed reactor system, etc., depending on whether the metal ore to be reduced is present in lump or fine-particle form or is needed for further processing to pig iron, pig iron-like products or for steel production.
The material to be reduced (e.g. metal ore, iron ore, iron oxide, etc.) is introduced in direct reduction in lump form—e.g. as lump ore or pellets—or in fine particle granular form into the plant for producing direct-reduced metal ores including at least one reduction unit. A process gas, which is also referred to as reduction gas, then flows through the material in the reduction unit using what is known as the countercurrent principle. In this case the material is for example completely or partly reduced by the reduction gas—iron oxide is reduced for example to sponge iron—and the reduction gas is oxidized in this process. The material reduced in the reduction unit by the reduction gas then for example has a degree of metallization of around 45 to over 95%.
The process gases necessary for the reduction of the metal ores (e.g. reduction gas) are for example obtained in a melter gasifier in a smelter reduction method (z.B. COREX®, FINEX®, oxygen blast furnace, etc.) or in a coal gasifier. The reduction gas created for example in a melter gasifier is preferably at a temperature of 750 to 1000° C. and is mostly dust-retaining and also laden with carbon monoxide and hydrogen (e.g. with around 70 to 95% CO and H2). The reduction gas is preferably introduced in the central to lower quarter of the reduction unit or in a fluidized bed reactor system, into the first fluidized bed reactor. It then rises upwards in the reduction unit and in a fluidized bed reactor system is conveyed from reactor to reactor and during this process reduces the material (e.g. metal ore, iron ore, iron oxide, etc.) in the countercurrent. The reduction gas is then tapped off from the reduction unit as so-called top gas or off gas.
Thereafter the reduction gas is cleaned in a gas cleaning device (e.g. gas washer) and possibly passed on as so-called recycle gas for removal of carbon dioxide CO2 to a device connected upstream of the reduction unit for separation of gas mixtures with associated compressing device. Pressure adsorption systems such as e.g. Pressure Swing Adsorption (PSA), Vacuum Pressure Swing Adsorption (VPSA) are especially used as the system for separation of gas mixtures, since these systems are more cost-effective by comparison with other adsorption methods. After its treatment in the device for separation of gas mixtures the recycle gas can then be introduced again as reduction gas into the reduction unit.
As well as the part recycling of the process gas used in the plant for example, so-called export gas which is obtained for example from methods of pig iron production or from a melt reduction process such as the COREX® or FINEX® process for example, is used for the reduction process in the reduction unit. The term “export gas” is used in particular to designate that part of the so called top gas which is withdrawn from the melt reduction process or the method of pig iron production, cooled as a rule and also de-dusted (wet or dry), and possibly for further process gases such as for example surplus gas from the melter gasifier. In such cases the used reduction gas from a blast furnace, a melter gasifier or a reduction bed/packed bed reactor is usually referred to as top gas.
If the export gas from one or more melt reduction plants (e.g. plants based on the COREX®- and/or FINEX® process) is drawn off for the production of direct-reduced metal ores, then such a plant is referred to as a combined cycle plant. The export gas is introduced in such cases into a supply line in the plant for producing direct-reduced metal ores or iron and used in the respective reduction unit such as for example fluidized bed reactor system, reduction bed furnace such as e.g. MIDREX® reduction bed, etc.). During introduction the supplied export gas can mix with the recycle gas of the plant for producing direct-reduced metal ores, wherein the supply line for the export gas usually emerges in front of the device for separation of gas mixtures into a line for the recycle gas of the plant for producing direct-reduced metal ores.
For optimum functioning of the reduction process in the plant for producing direct-reduced metal ores or iron however a CO2 removal which functions well in the device for separation of gas mixtures from the process gas—i.e. a mixture of recycle gas of the plant and supplied export gas—is necessary. Therefore the input pressure for the device for separation of gas mixtures is usually increased with the aid of the associated compressing device (e.g. one or more compressors). The input pressure at the device for separation of gas mixtures should usually lie at a constant input pressure of around 3 to 8 bar excess pressure, in order to guarantee an efficient and economic full CO2 removal from the process gases. Thus for example at least around 6 bar excess pressure is needed for CO2 removal by a PSA and at least around 3 bar excess pressure by a VPSA.
However the reduction unit used requires a significantly lower operating pressure. Thus a MIDREX® reduction shaft as the reduction unit for example only needs an input pressure of around 1.2 to 1.5 bar. Thus a pressure difference is produced by on the one hand the relatively high operating pressure of the device for separation of gas mixtures, especially when a PSA plant is used, and on the other hand the relatively low operating pressure of the reduction unit, which must be reduced for an optimal functioning of the system for production of direct-reduced metal ores.
Usually this pressure difference is reduced by a system of regulation valves, which is especially fitted between the device for separation of gas mixtures and the reduction unit or a gas heating device connected upstream of the reduction unit. However a reduction in the pressure difference using regulation valves has the disadvantage that this pressure difference remains unused or that—especially for the input pressure level at the device for separation of gas mixtures—energy must be expended, which will be destroyed unused thereafter. Thus for example when a device for pressure swing absorption (PSA) or a device for vacuum pressure swing absorption (VPSA) and a MIDREX® reduction shaft are used as the reduction unit, because of the necessary operating pressure, approximately 1 to 6 bar is destroyed unused. Thus the specific energy or power consumption of a combined cycle plant is relatively high and thus reduces its cost effectiveness. Noise generated by the valve station is also relatively high.
Furthermore the export gas, which is introduced at least partly as process gas into the plant for producing direct-reduced metal ores, can originate from for example from two plants for production of pig iron or melt reduction plants, which for example have different pressure levels at the respective transition point. This can be caused for example by different system pressure requirements at the respective melter gasifier. In order to obtain an approximately constant input pressure level for the reduction unit or the device for separation of gas mixtures, a bypass volume pressure regulation can be provided in these types of combined-cycle plants. This has the disadvantage however that approximately 10 to 20% of the export gas for use in the combined-cycle plant, especially in the plant for producing direct-reduced metal ores, is lost, whereby the productivity and the cost effectiveness of the plant is likewise reduced.